I'm leaning towards the pro, because my problem is really slow processing speed that can't keep up with the newer operating systems, browsers, and video requirements. I looked at an Air, but it has the exact same processor speed as my current computer!
Anne, it might not be the processor but that your old Mac doesn't have enough RAM or you need to update to Snow Leopard. I did both this past spring and it's like I have a new computer. I doubled the RAM last spring by buying more memory from macsales.com after my computer guru suggested it ("I'd be happy to charge you my hourly rate to come over and do it, but really, you can do it yourself").
He told me to call their customer service dept to determine exactly what I needed and to watch the how-to videos. The only tool I needed was a specific screwdriver -- a Phillips 00, as I recall (YMMV depending on whether you're doing a laptop or a desktop). We had one around the house but if you don't, you can buy one at a Radio Shack, hardware store, or order when you order the memory. Or borrow one from the university's computer center!
Also, there might be a bunch of stuff in the library that could be deleted to free up some space on the hard drive--old downloads especially--that make things work faster.
Upgrading to Snow Leopard was quick; the only wrinkle I remember* was having to find new drivers for my printers. The HP inkjet went very smoothly but it took a couple of calls to Brother tech support to get the laser printer to play nice. The good thing is that the interface with the printer was improved so much that it's now, finally, a good printer!
*I have a dim memory that there was one other issue but, as with labor during childbirth, I've blocked out the memory...
The reason I'm going to get a new computer is that, even though this one is mostly rebuilt (including keyboard...!) I want something lighter to take to coffee shop/library/vacation, ideally one with no financial info/bookmarks on it.